Remove HEADLESS option (emscripten-core#24537)

Closes emscripten-core#20554.

---------

Co-authored-by: Alon Zakai <[email protected]>

Author: RReverser

ChangeLog.md

@@ -22,6 +22,16 @@ See docs/process.md for more on how version tagging works.
 -----------------------
 - The `ENVIRONMENT` setting will now be automatically updated to include
   `worker` if multi-threading is enabled. (#24525)
+- Removed the `HEADLESS` option. It tried to simulate a minimal browser-like
+  environment before browser engines had real headless modes. For headless
+  testing, users are now encouraged to use
+  [Playwright](https://playwright.dev/), [Puppeteer](https://pptr.dev/) or
+  Node.js with [JSDOM](https://github.com/jsdom/jsdom) instead. You can also
+  use browser headless mode with `emrun` as follows:
+    emrun --browser=chrome --browser-args=--headless [..]
+  for chrome, or
+    emrun --browser=firefox --browser-args=-headless [..]
+  for firefox. (#24537)
 
 4.0.10 - 06/07/25
 -----------------

eslint.config.mjs

@@ -46,8 +46,6 @@ export default [{
     'src/cpuprofiler.js',
     'src/memoryprofiler.js',
     'src/gl-matrix.js',
-    'src/headless.js',
-    'src/headlessCanvas.js',
     'src/source_map_support.js',
     'src/Fetch.js',
     'src/settings.js',

site/source/docs/tools_reference/settings_reference.rst

@@ -1841,20 +1841,6 @@ testing.  See test_chunked_synchronous_xhr in runner.py and library.js.
 
 Default value: false
 
-.. _headless:
-
-HEADLESS
-========
-
-If 1, will include shim code that tries to 'fake' a browser environment, in
-order to let you run a browser program (say, using SDL) in the shell.
-Obviously nothing is rendered, but this can be useful for benchmarking and
-debugging if actual rendering is not the issue. Note that the shim code is
-very partial - it is hard to fake a whole browser! - so keep your
-expectations low for this to work.
-
-Default value: false
-
 .. _deterministic:
 
 DETERMINISTIC